Re: [RFC PATCH] configure: deprecate 32 bit build hosts

["Alex Bennée" <alex.bennee@linaro.org>]

Alistair Francis <alistair23@gmail.com> writes:

> On Thu, Sep 26, 2019 at 8:41 AM Alex Bennée <alex.bennee@linaro.org> wrote:
>>
>>
>> Thomas Huth <thuth@redhat.com> writes:
>>
>> > On 26/09/2019 15.46, Christian Borntraeger wrote:
>> >>
>> >>
>> >> On 26.09.19 14:58, Daniel P. Berrangé wrote:
>> >>> On Thu, Sep 26, 2019 at 08:50:36AM +0100, Peter Maydell wrote:
>> >>>> On Thu, 26 Sep 2019 at 00:31, Alex Bennée <alex.bennee@linaro.org> wrote:
>> >>>>>
>> >>>>> The 32 bit hosts are already a second class citizen especially with
>> >>>>> support for running 64 bit guests under TCG. We are also limited by
>> >>>>> testing as actual working 32 bit machines are getting quite rare in
>> >>>>> developers personal menageries. For TCG supporting newer types like
>> >>>>> Int128 is a lot harder with 32 bit calling conventions compared to
>> >>>>> their larger bit sized cousins. Fundamentally address space is the
>> >>>>> most useful thing for the translator to have even for a 32 bit guest a
>> >>>>> 32 bit host is quite constrained.
>> >>>>>
>> >>>>> As far as I'm aware 32 bit KVM users are even less numerous. Even
>> >>>>> ILP32 doesn't make much sense given the address space QEMU needs to
>> >>>>> manage.
>> >>>>
>> >>>> For KVM we should wait until the kernel chooses to drop support,
>> >>>> I think.
>> >>>
>> >>> What if the kernel is waiting for QEMU to drop support too ;-P
>> >>
>> >> For what its worth on kvm/s390 we never cared about implementing
>> >> 32 bit.
>> >
>> > Looking at tcg/s390/tcg-target.inc.c :
>> >
>> > ...
>> > /* We only support generating code for 64-bit mode.  */
>> > #if TCG_TARGET_REG_BITS != 64
>> > #error "unsupported code generation mode"
>> > #endif
>> > ...
>> >
>> > ... it seems to me that TCG does not support 32-bit on s390 either. I
>> > think we can remove s390 (32-bit) from the list completely?
>>
>> It's the same for riscv32 I think.
>
> I think riscv32 host *should* work, although I don't think it was ever
> tested.

You certainly don't support oversized guests:

  /* We don't support oversize guests */
  QEMU_BUILD_BUG_ON(TCG_TARGET_REG_BITS < TARGET_LONG_BITS);

>
> Considering that we have enough trouble keeping the riscv32 kernel
> booting it's probably ok to drop it from QEMU.
>
> Alistair
>
>>
>> >
>> >  Thomas
>>
>>
>> --
>> Alex Bennée
>>


--
Alex Bennée